Studies have been made to synthesize condensates through hydrolysis-condensation reactions using sol-gel methods in which silane compounds containing hydrolyzable silyl groups are used as starting materials.
Such condensates have been used in hard coating agents for plastic or ceramic materials, protective films for liquid crystal display elements, electric insulating materials for semiconductors, curing agents for coating materials, and other applications.
Compounds containing silicon are exceptional among metal alkoxide precursors in that they are less likely to undergo hydrolysis and condensation. For this reason, in condensation reactions of silane compounds containing hydrolyzable silyl groups, acids or bases are used as catalysts to accelerate the reactions to shorten the reaction time.
For example, Patent Literature 1 discloses a method for producing an alkoxysilane condensate from a tetrafunctional alkoxysilane such as tetraethoxysilane as a starting material in the presence of an acid catalyst. Patent Literature 2 also discloses a production method including hydrolyzing and condensing a trifunctional organic alkoxysilane such as vinyltrimethoxysilane in the presence of a basic catalyst.
As described above, a typical sol-gel method uses an acid or base catalyst to accelerate hydrolysis and condensation. However, many acid and base substances are corrosive substances which require technical considerations on corrosion of reaction equipment or storage equipment. In practice, the method often needs removal or neutralization of such an acid or base substance after the synthesis. However, it is not easy to completely remove the acid or base, and that even in the case of neutralization, the process becomes complicated and the amount of impurities is increased (see Patent Literature 3).
When a silane compound containing a highly active organic functional group such as an epoxy group is hydrolyzed and condensed, the acid or base catalyst used may destroy and deactivate the organic functional group.
Patent Literature 4 reports a technique by using a neutral fluoride salt as a catalyst. However, the term “neutral” as used in this patent literature does not mean that the pH of an aqueous solution of the fluoride salt itself is neutral, but merely means that the counter ions of the fluoride ions generated by dissolution of the fluoride salt in water are ions other than hydrogen ions. In fact, many aqueous fluoride salt solutions are weakly acidic. Also, many fluoride salts are known to generate highly toxic hydrofluoric acid in acidic aqueous solutions. Furthermore, silanols formed in the reaction may accelerate the generation of hydrofluoric acid.
On the other hand, in low reactivity conditions as in the absence of catalysts, the condensation reaction proceeds slowly and the hydrolysis and condensation take much time. Thus, it is difficult to obtain a condensate of a silane compound containing an organic functional group with low hydrolysis resistance, such as an epoxy group, without deactivating the organic functional group. Also, during the reaction, the presence of silanols may acidize the system to accelerate the hydrolysis of the organic functional group.